All patents, patent applications, and publications cited within this application are incorporated herein by reference to the same extent as if each individual patent, patent application or publication was specifically and individually incorporated by reference.
Microelectrodes and microelectrode arrays are useful, for example, in microelectronics, semiconductor chips, integrated microanalysis chips (i.e., “labs on a chip”), electro-optics, and electro-chemical nucleic acid arrays. One conventional method of fabricating a microelectrode is to photolithographically pattern photoresist on a metal thin film, and then wet etch areas of metal to leave the behind the electrode. One disadvantage to this approach is that certain critical dimensions such as combinations of electrode spacing and thickness (height) may be difficult to achieve because wet etching is an isotropic process. Another disadvantage is that the electrode walls after wet etching can be relatively rough. Dry etching, which can be anisotropic and used to produce less wall roughness, is difficult to achieve on the most useful electrode metals like gold.
The “lift-off” technique has been used as an alternative to wet etching. The lift-off technique involves patterning a photoresist on a surface then depositing a metal on the surface and the photoresist and then stripping the gold covered photoresist to leave behind the metal electrode. This technique usually requires a directional deposition of the metal since conformal coverage on the photoresist walls will not allow the stripper to dissolve the photoresist. Evaporation is typically used for directional deposition, but this is difficult to utilize in high volumes.
Physical vapor deposition (PVD) of metals, which is well proven in the semi-conductor industry, is difficult to use in a lift-off process since the coverage in conformal. PVD can be used with a “two-layer” photoresist lift-off technique. In a two-layer process, the walls of the bottom layer of photoresist are recessed compared to the walls of the upper layer. The PVD conformal coverage occurs on the walls of the upper layer of photoresist and the surface. A gap is left near the recessed walls of the lower layer, thereby allowing the stripper to dissolve and remove the photoresist, leaving behind the metal electrode. One disadvantage to this approach is that the walls of the electrode can be slanted significantly.